It is frequently desirable to be able to provide an indication whether a product has been exposed to an undesirable time-temperature history which results in substantial degradation of the product, or to a correct time-temperature history which may be required during processing or use of the product. This applies, e.g., to perishables such as foods, pharmaceuticals, and photographic products, which generally have limited useful lifespans which may be significantly shortened by exposure to undesirably relative high temperatures for a specific time period during storage, distribution, or use. This also applies, e.g., to certain products such as canned goods and biomedical materials which may be required to be held at certain high temperatures for a specific time period, e.g. to guarantee sterilization.
The rate of degradation, or other change in a product, at a given temperature is typically product dependent, i.e. some types of products show a greater increase in the rate of change for a given temperature increase relative to other products. Accordingly, it would be desirable to be able to provide indicators for use with various products which supply a visual indication of cumulative thermal exposure in which the rate of providing the visual indication of cumulative thermal exposure can be approximately matched to the rate of cumulative change, such as degradation, of the specific product to be monitored.
A number of systems have been described for providing indicators useful in detecting whether a product has been exposed either to specific time-temperature combinations or simply to a particular temperature which results in substantial degradation. Representative systems are disclosed, e.g., in U.S. Pat. No. 3,999,946 (use of compositions containing at least two conjugated acetylene groups which exhibit sequences of irreversible color changes at combinations of times and temperatures specific to each composition) and WO 96/28714 (use of viscoelastic material which migrates into a diffusely light-reflective porous matrix at a rate which varies with temperature to progressively change the light transmissivity of the porous matrix). A drawback of many of the indicators described in the prior art, however, is that they may frequently be useful only over a very limited temperature range, that they may not be easily designed to match the rate of cumulative change desired to be monitored for a particular product, that they may be bulky or expensive, that they may require cumbersome manual activation steps, or that they may depend upon diffusion or complex reaction mechanisms for their operation.